The present invention relates to a method of purifying exhaust gas emitted from internal combustion engines of vehicles by the action of a catalyst. More particularly, the present invention relates to an exhaust gas purifying method which is suitable for removing nitrogen oxides contained in an exhaust gas emitted when the internal combustion engine operates at a high air fuel ratio at which the fuel mixture is lean due to excessive oxygen.
Exhaust gas emitted from internal combustion engines of vehicles contains oxides, including nitrogen oxides, which are harmful to humans and cause environmental damage via such phenomenon as acidic rain. Therefore, various studies have been conducted into a method of purifying nitrogen oxides in exhaust gas.
At present, the air fuel ratio A/F (a ratio by weight of air A to fuel F) in gasoline engines for vehicles is set to about a stoichiometric ratio (A/F=14.5), and exhaust gas produced therefrom is purified by reducing nitrogen oxides to nitrogen and by oxidizing hydrocarbons and carbon monoxide, using a three-way catalyst having noble metals (rhodium, palladium platinum) as its main components.
Furthermore, concerning automobiles, recently, there has been significant development in lean burn engines in which the air fuel ratio is made larger than the stoichiometric ratio (A/F 14.5) to reduce fuel consumption, and such lean burn characteristics are expected to be used widely in the future. However, since a lean burn engine emits exhaust gas which contains oxygen of 0.5 vol % or more (hereunder, exhaust gas which contains 0.5 vol % or more oxygen is referred to as an oxygen containing exhaust gas), during purification of the exhaust gas by the three-way catalyst used at present, oxidization of hydrocarbons and carbon monoxide mainly occurs, but it is impossible to effectively reduce the nitrogen oxides.
Current diesel engines for vehicles operate at a high air fuel ratio having a more excessive level of oxygen than conventional diesel engines. Therefore, the above-mentioned three-way catalyst can not be used for purifying the exhaust gas of such diesel engines. At present, there is a NOx method to effectively reduce nitrogen oxides, but it has a drawback.
One of the current methods of removing nitrogen oxides uses NH.sub.3 and a V.sub.2 O.sub.3 --TiO.sub.2 catalyst. This method is characterized in that, even if a lot of oxygen coexists with nitrogen oxides in the exhaust gas, the nitrogen oxides can be removed. However, since the method uses NH.sub.3 which is a harmful substance and requires a NH.sub.3 supply tank, it is difficult to use this method in the internal combustion engines of vehicles, such as automobiles.
Recently, a lot of studies have been made of catalysts for purifying nitrogen oxides with excessive oxygen present, without using NH.sub.3. Of those methods, attention has been given to a method of removing nitrogen oxides, making use of hydrocarbons and oxygen contained in the exhaust gas. Of such catalysts at present, there has been reported a catalyst in which copper is supported on zeolite (which is disclosed in JP A 1-151706 and a 70th Shokubai-Toronkai Yokoshu(70th Catalyst-Symposium Preliminary Reports)3F 108 (1992)), a catalyst containing a rare earth metal, copper and/or rhodium (JP A 4-219147), and a catalyst comprising barium oxide, lanthanum or lanthanum oxide and platinum (JP A 5-261287).
However, the above-mentioned catalysts and exhaust gas purifying methods using the catalysts have the following problems.
The catalyst in which copper is supported on zeolite has a peculiar adsorption of nitrogen oxides by the copper, so that it has a high performance in a relatively low temperature zone. However, the temperature zone (temperature window) enabling catalytic action is narrow. Therefore, a study has been made to find a way of expanding the temperature window, and a catalyst including cobalt, a rare earth metal, copper, and rhodium in addition to zeolite, for example, has been proposed. However, zeolite-based catalysts have the problem that catalytic activity is lowered with the coexistence of a lot of water. A study has been made of this problem, and a method of improving drastically the resistance of zeolite (which is a support) against water and heat has at present still not been found. A catalyst used for purifying the exhaust gas of internal combustion engines in vehicles operated at a high air fuel ratio, as in lean burn engines, has to achieve a high nitrogen oxide purifying performance, in a wide temperature zone, and also has to have a high durability.
The catalyst, comprising barium oxides, lanthanum oxides and platinum, absorbs nitrogen oxides during the operation time of the lean burn engine and desorbs during the operation time of the engine at a stoichiometric ratio, to reduce the nitrogen oxides. Although the catalyst is based on a clever idea, it has a problem of thermal deterioration.
Further, it is desirable for exhaust gas purifying catalysts to be active both during a lean burn operation time and an operation time at a stoichiometric ratio, that is, to be both effective exhaust gas containing oxygen of more than 0.5 vol % and exhaust gas containing oxygen of 0.5 vol % or less.